In this section, we discuss several aspects of related work, including background and conventional technologies.
Ionic liquids by definition are salts that have melting points below 100 degree C. Interest in ionic liquids has grown markedly in recent years because of their potential applications in a wide range of fields including, Electroplating, Lubricant, Antistatic coating, Cleaning, Powder coating, Fire resistant treatment, Electrolytes in supercapacitors, fuel cells, lithium ion batteries, and lithium batteries, separations techniques such as Liquid-liquid extraction, Treatment of nuclear waste, Desulfurization of Diesel, Metal extraction, Gas purification and Membranes, solvents or reaction medium in Organic reactions, Acid catalysis, Immobilization catalyst in Synthesis of nanoparticles, in biotechnology applications such as Biomass conversion, Protein purification and Enzymatic reactions, Matrices of mass spectroscopy, and Chromatography and solvents for carbon dioxide capture, sulfur dioxide capture and hydrogen sulfide capture.
Ionic liquids are mainly composed of organic cations, such as alkylammonium, alkylphosphonium, alkylsulfonium, 1,3-dialkylimidazolium, alkyltriazolium, alkylpyridinium, etc. and mononuclear anions, such as BFsub.4, PFsub.6, CFsub.3SOsub.3, (CFsub.3SOsub.2)sub.2N, methide, CFsub.3COsub.2. Some ionic liquids containing non-fluoroanions, such as nitrate, perchlorate, alkyl sulfate and alkyl oligoether sulfate, dinitramide, amino acid anions. A variety of organic anions, have also been synthesized and studied. The chemical structure of the typical cations and anions comprised by ionic liquids are provided in FIG. 1
The chemistry of cation and the anion determines the physical and chemical properties of an ionic liquid. Therefore, it is possible to achieve specific physical property by choosing the proper combination of a cation and an anion. For example, the viscosities can be adjusted over a wide range of less than 50 cP to greater than 10,000 cP.
Carbon Dioxide Absorption by Ionic Liquids
There are two types of ionic liquids currently pursued in research field (1) Room temperature ionic liquids (RTILs), and (2) Task-specific or functionalized ionic liquids. High-pressure phase behavior of carbon dioxide with a variety of ionic liquids was first reported back in 2001 by Blanchard et al. Their study included ionic liquids, 1-n-butyl-3-methylimidazolium hexafluorophosphate, 1-n-octyl-3-methylimidazolium hexafluorophosphate, 1-n-octyl-3-methylimidazolium tetrafluoroborate, 1-n-butyl-3-methylimidazolium nitrate, 1-ethyl-3-methylimidazolium ethyl sulfate, and N-butylpyridinium tetrafluoroborate. The researchers observed that a large quantity of carbon dioxide could be dissolved in the ionic liquid phase.
This research group also latter showed that ionic liquids with the bis(trifluoromethylsulfonyl) imide anion had the largest affinity for carbon dioxide regardless of whether the cation was imidazolium, pyrrolidinium, or tetraalkylammomum. These results suggest that the nature of the anion has the most significant influence on the carbon dioxide solubility. The solubility of carbon dioxide in a series of imidazolium-based room-temperature ionic liquids has been determined by Baltus et al.
With the aim of finding ionic liquids that improve carbon dioxide solubility and to understand how to design carbon dioxide-philic ionic liquids, Muldoon et al. studied the low- and high-pressure measurements of carbon dioxide solubility in a range of ionic liquids possessing structures likely to increase the solubility of carbon dioxide. They examined the carbon dioxide solubility in a number of ionic liquids with systematic increases in fluorination. They also found that the anion plays a key role in determining carbon dioxide solubility in ionic liquids in agreement with other research reports.
Thus the literature reports indicate that fluoride containing anions bis[trifluoromethyl sulfonyl] amide and tris (trifluoro methyl sulfonyl) methide [methide] are most suitable as anions in the new ionic liquids for carbon dioxide capture.
The viscosity of common room temperature ionic liquids is quite high. For example, 1-n-butyl-3-methylimidazolium tetrafluoroborate (79.5 cP) is found to be 40 times more viscous as compared to 30 percent monoethanolamine solution at the same temperature (33 Cp). In order to meet the viscosity constraints, ionic liquids can be mixed with some common organic solvents or water. However, inclusion of such liquids will accompany their own drawbacks as well or this may be accomplished at the expense of decrease in gas capture ability. For example, addition of polyethylene glycol to an ionic liquid decreased the carbon dioxide solubility.
Without wishing to bind by any theory, based on the above discussion following conclusions can be arrived: (a) Room temperature ionic liquids themselves have shown adequate level of carbon dioxide solubility and (b) Mixing chemically absorbing species such amines, alcohols and amino alcohols with ionic liquids can shift the equilibrium towards higher carbon dioxide absorption even at low carbon dioxide partial pressures
Chemical Structural Features Crucial for Carbon Dioxide Absorption
There are over 10sup.18 ionic liquids available for exploration. It is not practical to synthesize every one of these compounds and select the best ionic liquid for carbon dioxide absorption/Therefore, ionic liquids containing cations in which amino and alcohol functional groups present in the same molecule was judiciously selected. The rationale behind this selection of these functional groups is discussed below.
The state-of-the-art technology for carbon dioxide capture is reversible chemical absorption into an aqueous amine solution. The capacity of an aqueous amine solution to chemically absorb carbon dioxide is a function of the route by which carbon dioxide reacts with the amine. There are two chemical routes generally considered for chemical absorption of carbon dioxide by amines.
Route 1 (carbamate formation—Amine:carbon dioxide=2:1)
Amines can react with carbon dioxide to form a carbamic acid (Rsub.2NCOOH).CARBON DIOXIDE+Rsub.2NH--->Rsub.2NCOOH (carbamic acid)
Depending upon its acidity, it may then give up a proton to a second amine molecule forming a carbamate (R2NCOOsup.−).Rsub.2NCOOH--->R2NCOsup.−+Hsup.+A
second amine molecule may be consumed by the proton liberated from carbamic acid forming carbamate.Rsub.2NH+Hsup.+--->R2NH2sup.+
Therefore for every carbon dioxide molecule, two amine molecules are used up. (2:1 ratio). Kinetically and thermodynamically this reaction pathway is generally favored for primary and secondary amines.
Route 2 (proton accepting base—Amine:CARBON DIOXIDE=1:1)
A second reaction route for carbon dioxide absorption is carbon dioxide hydration to form bicarbonate. In this pathway an amine molecule simply acts as a proton accepting base for the hydration of carbon dioxide. The overall stoichiometry for this second pathway iscarbon dioxide+water--->HCOsub.3sup.−+Hsup.+Rsub.3N+Hsup.+--->Rsub.3Hsup.+
According to the route 2, one mole of amine is consumed per mole of carbon dioxide, so in terms of absorption capacity it is more efficient. For tertiary and some sterically hindered primary and secondary amines this is the only pathway contributing to absorption. However, this pathway is generally less favorable kinetically than carbamate formation.
If the carbamic acid formed is a weak acid (higher pKa value) the extent of dissociation to form carbamate is low. Then the route 1 approaches a 1:1 carbon dioxide:amine molar stoichiometry because the carbamate does not deprotonate and consumes a second amine molecule. What type of amines can have higher carbon dioxide absorption capacity (1:1 carbon dioxide:amine ratio). This question was answered by Pauxty et al.
Pauxty et al. have studied the carbon dioxide absorption capacity of 76 different amines. Among these, seven amines, consisting of one primary, three secondary, and three tertiary amines, were identified as exhibiting excellent absorption capacities. Following discussion is based on the publication by Pauxty et al.
According to Pauxty et al. the most interesting result is that all of these amines, share a common structural feature, a hydroxyl group within 2 or 3 carbons of the amine functionality. While it is unclear what the role of this structural feature is, the distance of the hydroxyl functionality from the amine and the structural features around it appears crucial. For example, 2-piperidineethanol and 2-piperidinemethanol achieved capacities of near 1, whereas 3-piperidinemethanol only achieved a capacity of 0.8. This indicates that the proximity of the hydroxyl group and its freedom to move are important.
According to Pauxty et al. one possibility is that a hydroxyl group the appropriate distance from the amine functionality, and with the appropriate structural features surrounding it, is able to form a stable intramolecular hydrogen bond with the nitrogen to form a five or six member ring structure. Intramolecular hydrogen bond formation between amine and hydroxyl groups may decrease the amine pKa, for primary and secondary amines it may also destabilize carbamate formation and push the absorption toward the more stoichiometrically efficient route 2.
Therefore ionic liquids consisting of cations with hydroxyl groups at 2 or 3 carbon from amino groups have been synthesized.
Flame Retardant Ionic Liquids
Flame retardants for textile application have been reviewed by Weil and Levchik. They have provided historical details as well as current FR treatments of textile fabrics. Some of the common FR treatments to fabrics are summarized below based on this review article. Most common FR treatment of cotton fabrics is based on ammonium pyrophosphates. They impart self-extinguishing property to cotton fabrics. Borax is another common flame retardant agent used on fabrics. These treatments are temporary due to their solubility in water.
Polymers containing 35-45% bromine, poly(pentabromobenzyl acrylate) are used as a durable FR treatment on cotton and polyester fabrics. The FR property also can be improved by the addition of antimony oxide.
In recent years, halogen-free, low smoke, and fume flame-retardant composites are becoming of increasing importance, because halogen-type flame retardants can cause problems, such as toxicity, corrosion, and smoke. This has promoted the development of halogen-free, flame-retardant materials. Prior efforts have shown that metal hydroxides are nontoxic and smoke-suppressing additives with a high decomposition temperature in flame-retardant polymeric materials.
The FR material based on tetrakis(hydroxymethyl)phosphonium cation is the most widely sold commercial FR treatment product to date. It is generally agreed that ammonium and phosphonium salts have superior FR properties.
The above described FR treatments of fabrics are either non-durable or inefficient. Ionic liquids have excellent thermal stability and fire resistant properties. They are commercially available and also can be synthesized easily in an industrial scale.
The burning process consists of heating from an external source, decomposition of fabric, combustion of flammable chemicals released from the burning fabric, and propagation of flame.
Burn process starts from an external source of fire. When sufficient heat is applied the fabric starts decomposing. The pyrolysis of fabric (cellulose) results in the release of Levoglucosan and its volatile combustible fragments such as alcohols, aldehydes, ketones, and hydrocarbons. These flammable chemicals burn and propagate the flame and generate more heat. This process perpetuates until the fabric is completely consumed by fire. Part of the decomposition products from the fabric also produce a carbonized residue (char) that does not burn readily. The decomposition of cellulose can be expressed by the following equation:Cellulose→Flammable chemicals{↑}+Char{↓}(Uncatalyzed burning)
A flame retardant alters (catalyzes) the decomposition path of cellulose so that the amount of flammable chemicals is reduced and the amount of char formed is increased.Cellulose→Flammable chemicals{↓}+Char{↑}(Phosphonium catalyzed burning)
The ammonium and phosphonium flame retardants generally lower the decomposition temperature of cellulose and promote dehydration of the cellulose during thermal stress. Phosphorus-containing compounds increase the amount of carbon by redirecting chemical reactions involved in the decomposition. As more carbon is produced, the yields of volatile and flammable aldehydes and ketones are reduced. Ammonium based flame retardants also function through a similar mechanism.
In general, nylon fabrics have low flammability than cotton fabrics. Typical low weight nylon fabric melts and drips away, when exposed to flame and stops the propagation of flame.
Nylon Cotton (NYCO) fabrics are made using a 50% nylon/50% cotton blend and pirovide combat utility uniforms with excellent comfort and durability. However, NYCO fabrics have no flame resistant (FR) properties. Therefore for flame retardant fabrics one has to rely on expensive specialty fibers. Instead of using expensive fabrics, it will be economical to impart FR property on the NYCO fabric by treating them with flame resistant materials/coatings. The FR treatment should not deteriorate the fabric strength and should not add stiffness and significant weight to the fabric.
Ionic liquids containing ammonium and phosphonium cations exhibit exceptional flame resistant properties. In addition, they are non-flammable, high temperature stable (>250 degree C.), non-volatile liquids and amenable to coating on textile fabrics. Unlike conventional FR chemicals, ionic liquids are generally colorless and do not interfere with the other properties of the military fabrics such as camouflage. Along with flame resistant property ionic liquids also have added advantage of multi-functional capabilities such as antistatic, conductive and antimicrobial properties. In spite of these excellent multifunctional properties, ionic liquids are not widely used in fabric treatment due to the lack of detailed studies on the ionic liquid coatings on textiles.
Amino and hydroxy functional groups in the ionic liquid molecules can interact with the textile fabrics and can strongly bind to the fabric. This will increase the durability of the ionic liquids treated fabrics for several washings
Ionic Liquids as Electrolytes and Flame Retardant Additives to Electrolytes in Lithium Ion Batteries
Even though, energy storage capacity of lithium ion-batteries is superior to other rechargeable battery chemistries, safety issues related with the lithium-ion batteries are the major hindrance for their application as high power batteries. The low boiling organic solvents used as the electrolytes are the main cause of the safety concerns. These solvents have a flash point around 30° C. and could easily catch fire if vented from a hot battery. Moreover, the electrolytes decompose on contact with the charged active materials, both anodes and cathodes. At the end of the charging as well as at high temperatures, the cathode dissolves which accelerates the electrolyte decomposition. When a cell is heated above 130° C., exothermic chemical reactions between the electrolyte and electrodes trigger thermal run away reactions which are a serious safety hazard. Hence, high power lithium-ion batteries are developed with various external safety devices like current limiting devices, fuses, circuit breakers etc. These devices increase the cost and complexity of the battery module and also consume substantial power.
Considering these safety hazards, development of non-flammable, low volatile, thermally as well as electrochemically stable lithium battery electrolytes are essential for the use of high power lithium batteries in aviation. In this context, “ionic liquids” (ILs) which are liquids at room temperature composed of ions as the electrolytes for high power lithium batteries look extremely attractive. Pyrrolidinium based room temperature ionic liquids have been widely investigated as electrolytes in lithium batteries because of their low viscosities and reasonable conductivities. These ionic liquids are ‘non-flammable’ chemicals but are not ‘flame-retardants’. Uncontrolled thermal reactions in high-energy density lithium batteries may lead to fire and pyrrolidinium based ionic liquids cannot withstand these extreme conditions. This scenario undercuts the original reason for employing ionic liquids as electrolytes even by compromising on their low conductivity compared to organic carbonate based electrolytes. Therefore, alternate ionic liquids need to be developed which exhibit high ionic conductivity and non-flammability and are capable of quenching the fire in case of short circuits, local heating and or in abuse conditions such as overcharging.
Ionic Liquids for Corrosion Protection
Ionic liquids as desiccants and chloride removal system—Corrosion is a critical problem for the aircrafts. It costs Department of Defense over $10 billion year just in maintenance of equipment's and installations. Corrosion is not only a cost issue, but it also impacts our troop's readiness, safety and their performance. The effect of corrosion felt by the Air Force most because aircraft structures are mostly made of metal. Corrosion is usually battled with special alloys and a variety of corrosion protection coatings. However, there is no ‘silver bullet’ available to completely eliminate the corrosion problem. The corrosion issue can be alleviated if the environmental factors that hasten the corrosion of metal alloys can be addressed properly. Two important factors that affect metals in an aircraft are humidity and chloride content in the atmosphere. Currently humidity level in an aircraft is controlled with the help of dehumidifiers. However, chloride deposition on the aircraft parts requires special attention. Because, desiccants used in the humidity control system are not effective against chloride accumulation. Therefore, new efficient desiccants that not only dehumidify the environment but also remove chloride ions from air are needed.
Ionic liquid based desiccant systems are capable of both humidity control and chloride removal. Ionic liquids are non-volatile liquids as well as efficient desiccants. The ionic liquids can be functionalized to remove chloride ions from the environment.
It will be readily understood by the skilled artisan that numerous alterations may be made to the examples and instructions given herein. These and other objects and features of present invention will be made apparent from the following examples. The following examples as described are not intended to be construed as limiting the scope of the present invention.